FIG. 10 is a block diagram showing a schematic structure of one example of a conventional AM demodulator. This conventional AM demodulator is composed of the AM detection circuit 1, automatic phase control (“APC”) detection circuit 2, APC filter 3, phase shift circuits 4 and 5, voltage control oscillator (“VCO”) 6, and the low-pass filter 7.
Operation of this conventional AM demodulator will now be explained. An AM-modulated input signal is input into the AM detection circuit 1. The AM detection circuit 1 receives this input signal and a signal output from the phase shift circuit 4 multiplies the two input signals, and outputs the result of multiplication. The phase shift circuit 4 receives a signal output from the voltage control oscillator (VCO) 6 and shifts the phase of the received input signal by +45 degrees. The low-pass filter 7 separates a high-frequency component from a signal received from the AM detection circuit 1, and outputs the separated high-frequency component as a detection output.
The VCO 6 receives a control voltage output by the APC filter 3, and outputs a signal of an oscillation frequency according to the received control voltage. The APC filter 3 receives and smoothes a signal output by the APC detection circuit 2 so as to convert the received signal into a DC voltage. The APC detection circuit 2 receives the AM-modulated input signal and a signal output by the phase shift circuit 5. The APC detection circuit 2 compares phases of the two received signals and outputs a signal that indicates a difference between the two received signals. The phase shift circuit 5 receives a signal output by the VCO 6, and shifts a phase of the received signal by −45 degrees. Thus, the APC detection circuit 2, APC filter 3, phase shift circuit 5, and the VCO 6 constitute a PLL. This PLL generates a detection signal that is synchronized with a carrier of the received AM-modulated signal.
Particularly, in this PLL, the detection signal generated by the PLL, namely, the signal other than the AM-modulated input signal input into the AM detection circuit 1 has the same frequency and phase as the carrier of the AM-modulated input signal. As a result, the AM detection circuit 1 can separate a signal from the AM-modulated input signal that is synchronized with the detection signal, and the low-pass filter 7 outputs the separated signal as a wave to be modulated, namely, the detection output.
Television broadcasting or the like is not transmitted with a regular percentage modulation and is occasionally transmitted with a percentage modulation which exceeds 100% in some broadcasting stations. However, in the AM demodulator, when the input signal that has been over-modulated is input, there occurs a phenomenon that a detection output that is disadvantageously inverted is output.
This phenomenon will be explained in detail below. FIG. 11A to FIG. 11C are explanatory diagrams for explaining the inversion of the output which occurs at the time of over-modulation. When a waveform on the side lower than a center of the carrier (the broken line in the figures) is detected, as shown in FIG. 11A, since an amplitude of the wave to be modulated becomes larger than an amplitude of the carrier at over-modulation period T, a peak 100 of the wave to be modulated is in a position which exceeds the center of the carrier. Namely, in the AM demodulator, ideally, an envelope shown in FIG. 11B should be output as the detection output for the input signal (i.e. AM-modulated signal) shown in FIG. 11A.
However, since the phase of the carrier of the input signal is shifted by 180 degrees at a over-modulation period T, in the AM demodulator, the phases of the two signals received by the APC detection circuit 2 are also shifted by 180 degrees from the original states. The APC detection circuit 2 controls the VCO 6 so the original phase relationship is returned in order to follow the phase change of 180 degrees. As a result, a signal which is obtained by shifting the carrier of the input signal and the output of the VCO 6 by +45 are also shifted by 180 degrees.
In other words, in the conventional AM demodulator, in order to follow the phase change, the detection signal with a phase shifted by 180 degrees from a desired phase relationship is generated by PLL, and this detection signal is used for detection. As a result, as shown in FIG. 11C, a peak 101 in a position inverted with respect to the peak 100 at over-modulation period T of FIG. 11A is output as the detection output.
In order to prevent such inversion of the detection output, countermeasures are taken by providing means for stopping the PLL control at the time of over-modulation, or means for reducing speed of the PLL control. Because of provision of such means, following of the phase change due to the over-modulation can be prevented. However, with these countermeasures, the APC detection circuit 2 hardly operates at the time of over-modulation, and there arises a problem that stability at the time of over-modulation deteriorates.